The invention is generally related to medical isotopes and, more particularly, to a medical isotope production reactor.
Technetium-99m (t1/2 6.02 hr) is the most widely used radioisotope in nuclear medicine, accounting for more than 80% of all diagnostic nuclear medicine procedures. Technetium-99m (99mTc) is almost exclusively produced from the decay of its 66-hour parent 99Mo. Projected world demand for 99Mo by the year 2008 was estimated at approximately 11,000 to 12,000 Ci per week (6 days pre-calibrated). The most common method of 99Mo production is based on neutron irradiation in a nuclear reactor of a U—Al alloy or electroplated target enriched to 93 wt % 235U. After irradiation, the 99Mo is extracted from the other fission products by radiochemical methods. Although the specific activity achieved by this method is several tens of kilocuries per gram of molybdenum, large amounts of radioactive wastes are generated as byproducts of the fission process and the problem of long-lived fission product management is the major disadvantage in the production of 99Mo by this method.
The use of aqueous homogeneous solution reactors or water boiler reactors presents an attractive alternative to the conventional target irradiation method of producing 99Mo in that solution reactors eliminate the need for targets and can operate at much lower power than required for a target reactor to produce the same amount of 99Mo. Specifically, the use of solution reactors for the production of medical isotopes is potentially advantageous because of their low cost, small critical mass, inherent passive safety, and simplified fuel handling, processing and purification characteristics. These advantages stem partly from the fluid nature of the fuel and partly from the homogeneous mixture of the fuel and moderator.
In general, homogeneous reactor systems are superior to heterogeneous reactor systems in their inherent safety characteristics which arise from their greater radiolytic gas production per energy release, thereby resulting in a considerably larger prompt negative temperature coefficient of reactivity. However, the modularity of heterogeneous reactor systems provides a greater degree of freedom and versatility in the fuel arrangement. If practical methods for handling a radioactive aqueous fuel system are implemented, the inherent simplicity of a heterogeneous-homogeneous combinatorial reactor should result in considerable economic gains in the production of medical isotopes.
The advantages of utilizing homogeneous reactor technology for medical isotope production applications has prompted several countries, including the U.S., Russia, and China, to initiate programs to assess the feasibility of applying this technology on a commercial basis.
U.S. Pat. No. 5,596,611 discloses a uranyl nitrate homogeneous reactor (100 kW to 300 kW) for the production of 99Mo. The reactor is immersed in a containment pool which serves as a heat removal media for the sensible and decay heat generated in the reactor. The reactor vessel is finned to enhance the heat transfer to the containment pool. The reactor operates in a continuous mode in which the radioactive waste products are recirculated back into the reactor. A portion of the uranyl nitrate solution from the reactor is directly siphoned off and passed through columns of alumina to fix some of the fission products, including 99Mo, on the alumina. The 99Mo and some fission products on the alumina column are then removed through elution with a hydroxide and the 99Mo is either precipitated out of the resultant elutent with alpha-benzoinoxime or passed through other columns.
U.S. Pat. No. 5,910,971 discloses a small (20 kW to 100 kW) dedicated uranyl sulfate homogeneous reactor for the production of 99Mo which operates in a batch mode for a period of several hours to a week. After shutdown and following a cool-down period, the resultant solution is pumped through a solid sorbent material that selectively adsorbs the 99Mo. The uranyl sulfate and all fission products not adhering to the sorbent are returned to the reactor vessel. The reactor uses internal cooling coils for heat removal.
Although homogeneous reactor system concepts offer many advantages and greater flexibility for the production of 99Mo, potential power instabilities, which result from radiolytic bubble formation and thermal agitation, generate reactivity variations that can impair continuous stable operation. As a result, static solution reactor systems are power limited and, therefore, the specific activity of the 99Mo achievable, is limited by solution cooling constraints and potential thermal instabilities.